Commercially available diesel fuels are subject to a variety of regulatory requirements that constrain the properties of the fuel. For example, diesel fuels are typically required to meet one or more specifications related to cold flow properties.
Another category of requirements for diesel fuels is aromatics specifications, such as a restriction on the total number of polyaromatic hydrocarbons (PAHs) present in a diesel fuel. A polyaromatic hydrocarbon refers to a hydrocarbon containing two or more aromatic rings. Various types of hydroprocessing reactions can potentially be used to reduce the PAH content of a hydrocarbon feed.
U.S. Pat. No. 6,340,430 describes a method for processing a diesel boiling range feed. A diesel feed can be separated into a lighter and heavier portion. The feed is described as normally having a sulfur content of greater than 0.15 wt %. The heavier portion is catalytically dewaxed. The dewaxing catalyst may contain silicalite, MFI zeolites, or silicoaluminophosphates. Preferably, the dewaxing catalyst is a non-zeolitic molecular sieve essentially free of Y zeolite. The lighter portion is added to the dewaxed heavier portion prior to an aromatics saturation step. The aromatics saturation process is described as preferably using a hydrogen to hydrocarbon ratio of about 5,000 to 18,000 scf H2 per barrel of feedstock (about 890 to 3200 Normal, or standard, m3/m3). The dewaxing process is described as having a temperature at the inlet to the dewaxing process that is at least about 50° F. (30° C.) higher than the temperature at the inlet to the aromatics saturation zone.